@font: 100rem;

* {
    margin: 0;
    padding: 0;
}

#demo {
    width: 345/@font;
    height: 150/@font;
    background-color: skyblue;
    margin: 0 auto;
    margin-top: 15/@font;
    /* border: 0.01rem solid black; */
    border: 1px solid black;
}